This invention relates to an improved substrate suitable for use as the base of a lithographic printing plate, especially a plate useful for the production of continuous tone images.
Lithography or planographic printing is carried out by use of a printing plate with a substantially flat surface. The printing plate is chemically treated to bring about a printing surface, so that the printing area will accept oily ink and repel water, and so that the non-printing area will accept water and repel the oily ink. In performing the printing process the plate is moistened with water and inked, following which the plate is pressed in offset printing, against a rubber blanket which transfers the inked image to the paper being printed or in direct printing directly against the paper being printed.
A goal of lithographic printing is to produce extremely high quality images similar to those obtained from a photographic print. Conventional photographic materials have an average silver halide grain size of approximately one micrometer. To date, such a goal has not been reached.
Substantially all present day printing involving differences in tone from one part of the printed area to another is done by a halftone process. In accordance with this process separated solid areas like the elements of a stencil are what are printed. These solid areas are small dots of solid material which dots vary in size in direct relationship to the tones being matched. The dots are so small, however, that the presence of them is not distinguishable to the naked eye from a conventional viewing distance but their size variations create the optical illusion of variations in tonal values.
As is well known, the halftone process involves exposing the original copy to be duplicated through a camera lens and a cross-ruled glass or film screen. This screen, in some manner, breaks down the different tones into dots of varying size as just indicated. The photosensitive element or printing plate which eventually receives the screened image is then used to run off proofs for comparison with the original copy before the actual press run. Of course, when the copy to be duplicated is in color this must be broken down into three or four colors, each of the three or four colors being processed in the manner just described.
To date this halftone system has been about the sole one used, and it is almost universally used in photomechanical printing where large numbers of copies are desired. Nevertheless, it leaves much to be desired for a great deal of sharpness, color purity and detail are lost in utilizing halftone screens. The screens are approximately ten lines per millimeter, that is, a line separation distance of 100 micrometers for reasonable, high quality printing. All tones are degraded, and final proofs can never compare to the original copy.
A further disadvantage of the halftone process is the so-called moire effect which occurs if the orientation of the screen pattern is slightly different from the orientation of a regular pattern in the image and/or if two or more printed screened images are slightly misaligned during overlay printing. This affects reproduction of the picture in a manner that is usually not desired. A further disadvantage is that production of the necessary screen films is relatively expensive.
It is also known to print continuous tone pictures by the offset process from aluminum plates with fine-grained surfaces, without the use of a screen pattern. In this case printing is done "from the grain of the plate". The printing process is known as "screen-less planographic printing". This process is free from the three disadvantages mentioned above, but another disadvantage, causing great difficulties, is associated therewith, since it enables only relatively few shade gradations to be reproduced. Also, no procedure is yet known for obtaining printing plates in a reliably reproducible manner. Furthermore, the number of prints obtainable from a plate is relatively small.
Various methods are known in the art to produce a photosensitive coating which, when applied to a substrate and exposed through a continuous tone transparency, will produce a substantially continuous toned image, i.e., one which has a long tonal range or gray scale. However, the production of such a coating is in itself insufficient. One must examine the quality of each gray scale step for distinctness of the image and fineness of the grain within each step as well as the ability to use a plate having such a coating, for making thousands of consistent reproductions. It has been found that the surface topography of the substrate is a critical element to achieve this effect.